Aqua-ions reductants

The pentammine aqua ion [Ru(NH3)j(H20)]2+, best made by zinc amalgam reduction and aquation of [Ru(NH3)5C1]2+, undergoes extensively studied substitution reactions first order in both the ruthenium complex and the incoming ligand (e.g. NH3, py) and is a convenient source of other... 

None of the Cr(III) products from Equations 6 or 7 are effective crosslinkers since a chromic aqua ion must be hydrolyzed first to form olated Cr to become reactive. Colloidal and solid chromium hydroxides react very slowly with ligands. In many gelation studies, this critical condition was not controlled. Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). 

The resulting Tc(IV) aqua-ion is reduced at a high rate, e.g. under the action of pressurized molecular hydrogen [42,43], hydrazine [114] or other reductants (3) [115]. 

Well characterized in the solid state, the aqua ion probably exists as Rh2(H20) o It results from the reduction of mononuclear Rh(III) eomplexes ... 

The formation of [Fen(CN)5H20]3- as a product sets the basis for the catalytic processing of nitrite reduction by hydrazine under appropriate pH conditions. As shown in Fig. 13, nitrite binds to the aqua ion and rapidly converts to NO+. After the attack by N2H4, the adduct reorganization, associated with proton migration steps, favor... 

The Mo(IV) aqua ion was first reported by Souchay et al. as a monomeric species in 1966 (93), and the Mo3044+ core structure has been confirmed by single-crystal X-ray structure analyses, as described in this review, 95Mo NMR (94), EXAFS structure analysis (95), and 180-labeling experiments (96), after the appearance of many contradictory reports (97). Information on the reduction products of the Mo(IV) aqua ion is also available (98), Richens and Sykes have summarized the preparation of the different aqua ions of molybdenum in oxidation states II to V (99). 

Two methods are useful for the preparation of compounds with Mo-3O3S (1) reduction of Mo2C>3S2+ with NaBH4 (30) and (2) reduction of M02O3S2"1 with [Mode]3- (100). Compounds 1 and 2 in Table II were prepared from the aqua ion Mo30sS(aq)4+ and the corresponding ligands. 

Some preparative methods of the sulfur-bridged incomplete cubane-type aqua ion Mo3S4(aq)4+ have been reported (1) reduction of [Mo-202S2(cys)2]2 with NaBH4 (38), (2) reaction of Mo(CO)6 with Na2S (49), and (3) electrolysis of [Mo202S2(cys)2l2 (100). 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

BC is reasonably stable at neutral-to-alkaline pH but decomposes rapidly in acidic media. The reaction of [99mTc04]" with BC at pH 11-12 yields quantitatively the organometallic aqua ion [99mTc(OH2)3(CO)3]+. Mechanistically, it is possible that BC acts as a ligand which binds to the technetium centre. Hydride transfer followed or paralleled by reduction occurs concomitantly with CO coordination. The X-ray structure of a model with K+ as the metal is shown in Fig. 1. 

Shimizu et al. used simple rhodium-aqua ions (Rh3+) immobilized onto polymer-modified electrodes to perform the electrochemical reduction of NAD+ [114]. Rh3+ was loaded onto polymeric anion doped-polypyrrole membranes coated on the surface. Electrochemical reduction of NAD+ with immobilized Rh3+ was performed at —0.85 V, where Rh3+ was reduced to Rh+. NADH was produced without detectable formation of NAD-dimers. 

The greenish blue aqua ion [V(H20)6]3+ can be obtained as above or by electrolytic or chemical reduction of VIV or V solutions. Such solutions, and also others, of Vm are subject to aerial oxidation in view of the potential... 

The Aqua Ion. Electrolytic or zinc reduction of acidic solutions of Vv, Vlv, or V111 or dissolution of the metal in acid produces violet air-sensitive solutions containing the [V(H20)6]2+ ion. These are strongly reducing (Table 17-1) and are oxidized by water with evolution of hydrogen even though the standard potential V3+/V2+ would indicate otherwise. The oxidation of V2+ by air is complicated and appears to proceed in part by direct oxidation to V02+ and in part by way of an intermediate species of type VOV4+. 

The aqua ion has been extensively used as a reductant in studies on the mechanism of electron transfer reactions, best exemplified by the classical example of the reaction of Cr2+(aq) with [Com(NH3)5X]2+. This reaction proceeds via an inner-sphere (ligand-bridged) mechanism, as in the general reaction sequence... 

The aqua ion can be obtained by electrolytic or peroxosulfate oxidation of Mn2+ solutions, or by reduction of Mn04. The ion plays a central role in the complex redox reactions of the higher oxidation states of manganese in aqueous solutions. It is most stable in acid solutions, since it is very readily hydrolyzed ... 

Aqua ions are known but not very stable. Substitution of Pt in aqueous solution is sometimes zero-order in the added ligand, L, or can have both L-dependent and L-independent contributions to the rate, probably because intermediate formation of an unstable aqua complex is the rate-determining step for the L-independent pathway. A large number of O-donors, particularly anionic ones, give stable complexes, for example, carbonate, acetate, oxalate, acetylacetonate, and alkoxide. Tetrameric platinum(II) acetate is formed by formic acid reduction of Pt solutions in acetic acid. It does not appear to be a very useful synthetic precursor for Pt chemistry. The acetylacetonate [Pt(acac)2] is monomeric and square planar. 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

The zinc reduction of Eu + to Eu +, followed by its precipitation as the sulfate, is a traditional step in the separation of europium from other lanthanides. In general, the solubilities of the inorganic compounds of the Ln + ions resemble those of the corresponding compounds of the alkaline earth metals (insoluble sulfate, carbonate, hydroxide, oxalate). Both europium and the Sm + and Yb + ions can also be prepared by other methods (e.g. electrolysis), although these solutions of the latter two metals tend to be short-lived and oxygen-sensitive in particular. Eu + is the only divalent aqua ion with any real stability in solution. Several divalent lanthanides can, however, be stabilized by the use of nonaqueous solvents such as HMPA and THE, in which they have characteristic colors, quite distinct from those for the isoelectronic trivalent ions on account of the decreased term separations. 

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